Certain measurement applications require measuring the wavelength or frequency, or related shifts, of a radiation source to very high levels of resolution over a relatively small wavelength range. Examples include high resolution interferometric type encoders, various non-contact profilometer sensors, applications in the telecom industry, as well as general laboratory applications. In addition, for many applications, the measurement must be conducted within a small space and at a low cost. Several methods are commonly used for wavelength measurement, including spectrometers, interferometers, and transmission through optical filters.
FIG. 1A shows a simple measurement system 10 for measuring wavelength shift using an optical bandpass filter. The measurement system 10 includes an input incident beam 12, a bandpass filter 14, a filtered beam 16, and a power detector 18. The input incident beam 12 is filtered by the filter 14 to produce the filtered beam 16. In this application a bandpass filter is not strictly required, as any optical element having a non-negligible wavelength transmission dependence can be used. The power of the filtered beam 16 is detected by the power detector 18.
FIG. 1B shows the transmission spectrum for the bandpass filter 14. The filter 14 is characterized by a central wavelength λ0, as well as its full width half maximum (FWHM) wavelength Δλ. A point P is shown on the filter curve 20 at a wavelength X1 and a transmission level Y1. It can be seen that the point P is located on the steep part of the filter curve 20, and that slight changes in the wavelength can thus be sensed by measuring the change in the transmitted power, as is done by the power detector 18 of FIG. 1A. In this manner, once the filter curve 20 is established, the measurement system 10 of FIGS. 1A and 1B provides a simple configuration for determining a wavelength shift based on a transmitted power or intensity.
FIG. 2 illustrates a measurement system 30 which offers certain improvements over the measurement system 10 of FIG. 1A. As shown in FIG. 2, the measurement system 30 includes a beam splitter 34, a filter 38, and power detectors 42 and 46. An input incident beam 32 is split into two beams 36 and 44 by the beam splitter 34. The beam 36 is filtered by the filter 38 to produce the filtered beam 40. The power of the filtered beam 40 is detected by the power detector 42. The power of the beam 44 is detected by the power detector 46. By utilizing the outputs of the power detectors 42 and 46 to compute a ratio of filtered to non-filtered beam powers, deviations in the incident power are nominally eliminated as error sources. In other words, in contrast to the measurement system 10 of FIG. 1A which was unable to differentiate between wavelength shifts and power source fluctuations, the measurement system 30 of FIG. 2 uses a power ratio signal which is insensitive to deviations in the incident power, and thus more reliably discriminates wavelength shifts.
FIG. 3A illustrates a measurement system 50 which provides an alternative configuration for measuring wavelength shifts. Similar to the measurement system 30, the measurement system 50 utilizes the ratio between two power detectors to eliminate the incident power dependence to the first order. The measurement system 50 includes a beam splitter 54, filters 58 and 66, and power detectors 62 and 70. An incident input beam 52 is split into beams 56 and 64 by the beam splitter 54. The beam 56 is filtered by the filter 58 to produce a filtered beam 60. The power of the filtered beam 60 is detected by the power detector 62. The beam 64 is filtered by the filter 66 to produce a filtered beam 68. The power of the filtered beam 68 is detected by the power detector 70.
FIG. 3B illustrates two filter curves 80 and 82 which correspond to the filters 58 and 66, respectively. As shown in FIG. 3B, the filter curve 82 overlaps with the filter curve 80. In other words, the transmission spectrum of the filter 66 overlaps with the transmission spectrum of the filter 58. A point P1 is shown on the filter curve 80 at a wavelength X1 and a transmission power Y2, and a point P2 is shown on the filter curve 82 at the wavelength X1 and at a transmission power Y1. It will be appreciated that for wavelengths increasing from wavelength X1, that the transmission power on filter curve 80 is decreasing, while the transmission power on filter curve 82 is increasing. Thus, the ratio between a transmission power Y1 corresponding to the filter 66, and a transmission power Y2 corresponding to the filter 58 is unique for a particular wavelength over the wavelength transmission spectrum that is shared by the filter 58 and the filter 66. By utilizing the outputs of the power detectors 62 and 70 to compute a ratio of filtered beam powers, deviations in the incident power may be largely eliminated.
One prior art patent illustrating one of the prior art measurement systems is U.S. Pat. No. 4,308,456. The '456 patent describes a system utilizing two filters and two photodiodes, where the output of the first filter decreases with increasing wavelength, and the output of the second filter increases with increasing wavelength in a substantially linear manner over a bandwidth of 575–590 nm. In one embodiment, a beam splitter is utilized to split the incident beam into two beams, which are then each sent through a respective filter to a respective power detector. The ratio of the first and second outputs is taken and compared with predetermined values corresponding to preselected known frequencies for providing an output corresponding to the frequency of the light. The purpose of the invention is stated to be to provide an accurate measurement of the color of the light.
Another prior patent illustrating one of the prior art measurement systems is U.S. Pat. No. 5,729,347. The '347 patent teaches using a ratio between the filter output and a control signal. In one embodiment, an optical coupler is utilized to divide an incident beam into two beams, one of which is further divided by another opto-coupler into another two beams, for a total of three beams. The two beams divided by the second opto-coupler are each sent through respective filters, and are then processed by an amplifier and an A-to-D converter. The other beam is not filtered, but is also processed by an amplifier and an A-to-D converter. In the processing, the two digital values from the two filtered beams are evaluated, and one of the values is selected for further use. Then, a transmission ratio with respect to the non-filtered beam is computed, and a corresponding wavelength is output to a display. This provides a measurement system with a measurement of a wavelength in a relatively small predetermined range. The '347 patent also teaches forming a high resolution spectrometer by taking transmission ratio readings continuously while rotating the filter at progressive angles and performing suitable signal processing on the results to determine the spectrum of the incoming signal.
The present invention is directed to a method and apparatus that provides an improved wavelength measurement system. More specifically, the present invention is directed to a measurement system with a highly integrated structure that substantially reduces wavelength measurement or discrimination errors due to environmental variations. In addition, the present invention achieves these objectives in a compact and low cost configuration.